As a former HV fitter and driver I agree, not the best idea but at least in a plane by the time you get settled at 25,000ft the rear upper tank is gone and as per this report
http://www.spitfireperformance.com/Spitfire_IX_ML-186_Handling.pdf the handling is back to normal. I'm not arguing about whether the Spit could be an escort plane, it could never be a P51 but it's range could be easily tripled with the technology Supermarine had at the time.
Part of the argument is that range and operational radius are not the same thing for a fighter. Obviously radius is, in theory, 1/2 the range but
operational radius is NOT figured using max range of the aircraft using auxiliary tanks.
It is figured using the fuel capacity of the plane in combat condition.
Any none combat rated external fuel tanks are gone with whatever fuel was still in them.
The fighter must be in condition to at least make some sort of credible fight, A Spit MK XIV might haul a 90 gallon slipper tank around in a fight, a Spit MK V would be in serious trouble trying to do that. From power to weight, thrust to drag and structural strength standpoints. A MK VIII or IX falls where?
As the pilots
may tell us at these aft center of gravity situations it is not like flipping a switch. At some point it might be but I believe that there is a more gradual shift/change in flying characteristics. The Mustang is a real puzzle, somehow at 25 US gallons in the rear tank the plane is not only safe to fly, they want you to keep that 25 gallons in the rear tank as the reserve and use every other drop of fuel in the plane because the plane lands better (handles better) with 25 gallons in the rear tank, but at 26 gallons the plane is dangerous???
The RAF allowed the Mustang III to use 40imp gallons in the rear tank as the change over point from aceptable (my word, I am going from memory here) to not acceptable?
A wartime manual for the P-51 says things rather different than the post war manual.
For one thing they figured the stick reversal at 4 Gs rather than 6.75 and the manual is somewhat contradictory.
"When you are carrying more than 40 gallons of fuel in your rear tank do not
any acrobatics. The weight of this fuel shifts the center of gravity back so that the airplane is unstable for any but straight and level flight."
which is followed by
"Be sure you are accustomed to the changed flying characterics of the airplane before engaging in any maneuvers with full fuselage tank. You need at least one or two hours of flying with the airplane in this condition to accustom yourself to it."
Apparently acrobatics and maneuvers are not the same thing. Not sure where
straight and level falls
The manual is for planes fitted with tail warning radar and many of them had a 20lb bob weight fitted to the elevator controls. I have no idea if that really fixed things or if they only thought it did at time.
The manual also states (under fuel tank usage) that fuel return line from the carb went to the left wing tank on early planes and the rear fuselage tank on later planes, the return line could send
up to 10 gallons an hour depending on flight conditions back to the tank. If space was not available in the tank the fuel was vented overboard.
There is no need to try to warm up and tank off on auxiliary tanks as the main tank used for those purposes will be partially (or fully) refilled in flight.
apparently there was little problem in partially filling the Mustang rear tank in flight????
getting to another post
The argument is the Spit could not fly escort missions, my argument is just because it did doesn't mean it couldn't. Sydney Cotton had no issues fitting extra fuel in his PR spits like the under seat 20G tank, if the Spit was given to men like him, men that found solutions to problems instead of excuses the Spit could have had modifications to greatly improve it's range,
Photo recon planes were not expected to maneuver like a fighter, they were expected to use speed and altitude to evade interception so CG location was much less critical (or if you prefer lousy cg location was considered more acceptable for them).
Some of the "solutions" only became possible with the fitting of more powerful engines that used better fuel. And lets not forget better runways/flying fields. AN extra 500-1000ft of take-off distance to the proverbial 50 ft obstacle is a big help in getting a heavily loaded plane off the ground regardless of how much extra power the plane had.
A plane that has 250imp of fuel on board that crashed into the trees at the end of the runway isn't escorting anything anywhere.
We also have to see what the different Merlins were rated at for take-off power vs combat power and when (or if) the take-off power ratings were changed.
The Merlin 61 was rated at 1280hp for take-off at 12lbs, the V-1650-3 was rated at 1380hp at 15 1/4 lbs and the V-1650-7 was rated at 1490hp at 15 1/4 lbs, some of the later British Merlin's had increased take-off power settings. Using WEP power for take-off might have been possible but would have been frowned upon. A Manual for the P-51 says that each use and duration of WEP power had to logged and when the engine reached 5 hours of WEP use the engine ws to be pulled for a complete knock down inspection (this assumes the engine made it that far without particles in the oil or some other sign of distress).
Please note that I am not trying to Criticize the British Merlins or degrade the Spitfire here, simply noting that there were other considerations that somewhat depended on time that might have prevented or counted against a much earlier development of the escort fighter.
(end sarcasm)
